Hmmm I dunno, my diff is fine and its a 1994 R33 GTST, it still locks both fine.... Perhaps its worth taking out and shimming or at least inspecting
Barring a massive failure like Driveshaft/CV which I'm pretty sure youd notice lol, my only other guess is that the diff has become lazy and you might just be able to re-shim and fix it.... Simple things first but, check the fluid. Mine was a bit funnny when i first got it and i found the fluid was low. Changed it to sme new stuff and it was fine after that
If you want to change it, I use Castrol SAF-XA (or something like that) its relatively cheap and does the job. Id recomend trying that first if it is the diff stuffing up